[gn] port ade28a7 (clang-doc asset copy to share/clang)

Author: nico

llvm/utils/gn/secondary/clang-tools-extra/clang-doc/tool/BUILD.gn

@@ -1,6 +1,15 @@
+copy("assets") {
+  sources = [
+    "../assets/index.js",
+    "../assets/clang-doc-default-stylesheet.css",
+  ]
+  outputs = [ "$root_build_dir/share/clang/{{source_file_part}}" ]
+}
+
 executable("clang-doc") {
   configs += [ "//llvm/utils/gn/build:clang_code" ]
   deps = [
+    ":assets",
     "//clang-tools-extra/clang-doc",
     "//clang/lib/AST",
     "//clang/lib/ASTMatchers",